The present invention relates to an optical information medium capable of recording optically readable information; and, more particularly, to an optimum optical information recording medium adapted for use in recording and reproducing high-density optical information by using red laser beams having short wavelengths in the range from 630 to 670 nm.
A digital versatile disc (DVD) capable of recording and reproducing high-density information has been put into practical use by the help of the recent development and practical utilization of a laser having a short wavelength. In DVD, there are provided an information recording area on at least one main surface thereof, a plurality of pits retaining recorded information and formed on the information recording area, and a reflective layer formed of a metal film and provided on the whole information recording area.
In order to implement the recording and reproduction of high-density information in DVD, DVD standards are established, which are different from those of the compact disc (CD), currently the most widely used optical information medium. For instance, in order to prevent bending or warping of a disc (i.e., in order to decrease the influence of tilt from optical axis), the DVD employs a substrate having a thickness of 0.6 mm, which is approximately a half of that of the CD. In addition, for the dimensional compatibility with the conventional CD of 1.2 mm, the DVD adopts a structure in which two discs are bonded together. According to the standards of the DVD, one DVD disc is required to have a maximum recording capacity of about 4.7 GB, big enough to record video and audio information for an average running time of about 133 min.
In the DVD standards, there exist needs to develop a recordable optical information medium and a recording technique thereof, which has the physical and optical compatibility with read-only DVDs, such as a DVD-Video storing prerecorded video information and a DVD-ROM storing prerecorded computer program or data, and is capable of having the high-density recording capability of maximum 4.7 GB.
The DVD described above has a track pitch of 0.74 xcexcm and a minimum pit size of 0.4 xcexcm (or 0.44 xcexcm in the case of a DVD having dual recording layers); and, therefore, it is possible to achieve higher density recording in DVD than in CD having a track pitch of 1.6 xcexcm and a minimum pit size of 0.83 xcexcm.
The substrate of a recordable DVD is formed by, e.g., a suitable molding method such as an injection molding technique, wherein the substrate is conventionally formed through the use of a stamper whose groove shape is identical throughout the inner to the outer peripheral regions thereof. In injection molding the substrate using such a stamper, filling properties of a resin tend to be different from the inner to the outer peripheral region even in the case where molding conditions, such as pressure, resin temperature and the like, are optimally controlled. That is, since the track pitch and the width of the groove on the substrate of the recordable DVD are smaller than those on the substrate of the CD, the groove of the stamper used in forming the substrate of the recordable DVD can be fully filled with the resin at the inner peripheral region of the stamper, whereas the groove of the stamper may not be completely filled at the outer peripheral region thereof, resulting in deteriorated transferability of the stamper shape at the outer peripheral region. As a result, edges of the land at the outer peripheral region of the substrate fabricated as described above are bound to be rounded due to insufficient filling of the resin and thus sloping angles of the groove become smaller, resulting in the shallower and wider groove at that region.
In DVD, it is required that the difference xcex94PP between the maximum and the minimum amplitudes of the push-pull signals for proper tracking servo be within 15% of a reference value determined by the push-pull signals. The amplitude difference xcex94PP between push-pull signals required for accurate tracking servo may be defined as:
xcex94PP=(PPMAXxe2x88x92PPMIN)/(PPMAX+PPMIN)
wherein PPMAX and PPMIN represent the maximum and the minimum amplitudes of push-pull signals from a non-recording region of the DVD, respectively. However, in such an optical information recording medium having a land with rounded edges at the outer peripheral region, the push-pull signals from the outer peripheral region become smaller than those from the inner peripheral region. This is because ideal diffraction conditions cannot be met at the outer peripheral region where the groove is shallower and wider and sloping angles of the side surfaces of the groove are small due to the land with rounded edges.
It is, therefore, a primary object of the present invention to provide an optical information recording medium capable of recording optically readable high-density signals thereon, wherein amplitude differences between push-pull signals from an inner and an outer peripheral regions of the medium are reduced, thereby enabling accurate tracking to be performed throughout the inner and the outer peripheral regions.
Another object of the present invention is to provide a substrate capable of implementing a well-defined groove thereon, thereby enabling ideal diffraction conditions to be obtained.
Still another object of the present invention is to provide a stamper having an improved transferability for providing a substrate having a well-defined groove thereon.
In the course of addressing the drawbacks of the prior art, the inventors of the present invention have found that amplitudes of push-pull signals for tracking servo are determined depending on the stamper transferability to a substrate established at the time of injection molding the substrate. In particular, the inventors have found that the stamper transferability can be improved and a well-defined land and groove can be obtained by modifying the groove at the outer peripheral region of the stamper to have a shape to be easily filled with a resin.
Therefore, in accordance with the present invention, widths of the groove and sloping angles of the sides thereof are made to be different from the inner to the outer peripheral region, enabling the substantially identical stamper transferability to be obtained at the inner and the outer peripheral regions at the time of injection molding a light transmitting substrate. As a result, amplitude differences between push-pull signals for tracking servo can be reduced and accurate tracking servo can be performed throughout the inner and the outer peripheral regions of the optical information recording medium.
In accordance with the present invention, there is provided an optical information recording medium comprising:
a light transmitting substrate for transmitting a recording laser beam therethrough;
a tracking groove formed on a surface of the light transmitting substrate;
a recording layer formed on the surface and the tracking groove; and
a reflective layer, formed on the recording layer, for reflecting recording laser beam,
wherein optically readable signals are recorded by the recording laser beam irradiated through the light transmitting substrate.
Further, widths of the groove at the outer peripheral region of the light transmitting substrate are smaller than those at the inner peripheral region of the substrate.
By making the widths of the groove at the outer peripheral region of the light transmitting substrate smaller than those at the inner peripheral region thereof, the land at the outer peripheral region is widened and thus can be easily filled by a resin during injection molding. As a result, rounded edges of the land and the shallow and wide groove due to insufficient filling of the resin at the outer peripheral region of the light transmitting substrate can be prevented and thus amplitude differences of the push-pull signals for tracking servo due to varying diffractions of the laser beam at the inner and the outer peripheral regions can be also reduced.
Specifically, it is preferrable that a ratio of the width of the tracking groove at the outer peripheral region of the light transmitting substrate to that at the inner peripheral region thereof is greater than or equal to about 0.8. If the ratio is greater than or equal to about 0.8 but less than 1, the amplitude differences between the push-pull signals become within about xc2x115% of the average of the maximum and the minimum amplitude values of the push-pull signals, satisfying conditions required for accurate tracking servo.
In addition, in accordance with the present invention, the sloping angles of both sides of the tracking groove with respect to the plane of incidence of the recording laser beam at the outer peripheral region of the light transmitting substrate are to be smaller than those at the inner peripheral region thereof. Consequently, the groove at the outer peripheral region of the stamper can be more easily filled by the resin during injection molding. More specifically, the sloping angles at both sides of the tracking groove at the inner and the outer peripheral regions are within a range from about 55xc2x0 to about 75xc2x0 and, within this range, the sloping angles at the outer peripheral region are made to be smaller than those at the inner peripheral region.
In accordance with the present invention, a leveling index L(=1xe2x88x92B/A) is preferably to be within the range of about 0.2xe2x89xa6Lxe2x89xa6about 0.5 throughout the inner and the outer peripheral regions of the light transmitting substrate, wherein L represents a degree of leveling of the recording layer; A, a depth of the tracking groove on the substrate; and B, a depth of the recording layer, both being measured at an identical location (shown in FIGS. 4 and 5).
Therefore, as the leveling of the recording layer is more pronounced, i.e., as the recording layer becomes more flattened or planarized, the difference between the depth A of the pre-groove and the depth B of the recording layer becomes greater and thus the value L increases. If L is equal to zero, it means that the recording layer is not leveled at all and thus the depth A of the pre-groove is identical to the depth B of the recording layer. On the other hand, the value of L=1 means that the recording layer is perfectly leveled and thus the depth B of the recording layer is zero.
If L is within the range from about 0.2 to about 0.5, the reflectance of the optical information recording medium becomes higher and the amplitudes of the push-pull signals for the tracking servo remain favorable. On the other hand, if L is less than about 0.2, the reflectance of the optical information recording medium becomes low and if L is greater than about 0.5, it is difficult to obtain the desired amplitudes of the push-pull signals.